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The MessagePort interface of the Channel Messaging API represents one of the two ports of a MessageChannel, allowing sending of messages from one port and listening out for them arriving at the other.

Note: This feature is available in Web Workers.

Methods

Inherits methods from its parent, EventTarget

MessagePort.postMessage
Sends a message from the port, and optionally, transfers ownership of objects to other browsing contexts.
MessagePort.start
Starts the sending of messages queued on the port (only needed when using EventTarget.addEventListener; it is implied when using MessagePort.onmessage.)
MessagePort.close
Disconnects the port, so it is no longer active.

Event handlers

Inherits event handlers from its parent, EventTarget

MessagePort.onmessage
An EventListener called when a MessageEvent of type message is fired on the port—that is, when the port receives a message.
MessagePort.onmessageerror
An EventListener called when a MessageEvent of type MessageError is fired—that is, when it receives a message that cannot be deserialized.

Example

In the following code block, you can see a new channel being created using the MessageChannel.MessageChannel constructor. When the IFrame has loaded, we transfer MessageChannel.port2 (a MessagePort) to the IFrame using MessagePort.postMessage along with a message. The handleMessage handler then responds to a message being sent back from the IFrame (using MessagePort.onmessage), putting it into a paragraph.

var channel = new MessageChannel();
var para = document.querySelector('p');
    
var ifr = document.querySelector('iframe');
var otherWindow = ifr.contentWindow;

ifr.addEventListener("load", iframeLoaded, false);
    
function iframeLoaded() {
  otherWindow.postMessage('Hello from the main page!', '*', [channel.port2]);
}

channel.port1.onmessage = handleMessage;
function handleMessage(e) {
  para.innerHTML = e.data;
}   

For a full working example, see our channel messaging basic demo on Github (run it live too).

Specifications

Specification Status Comment
HTML Living Standard
The definition of 'Message ports' in that specification.
Living Standard  

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 4 (Yes) (Yes) 10.0 10.6 5
Available in workers (Yes) ? 41 (41) (Yes) (Yes) (Yes)
onmessageerror 60 ? 57 (57) ? 47 ?
Feature Android Webview Chrome for Android Edge Firefox Mobile (Gecko) Firefox OS (Gecko) IE Phone Opera Mobile Safari Mobile
Basic support (Yes) 4 (Yes) No support No support 10.0 11.5 5.1
Available in workers (Yes) (Yes) ? 41.0 (41) (Yes) (Yes) (Yes) (Yes)
onmessageerror 60 60 ? 57.0 (57) 47 ? 47 ?

See also

Document Tags and Contributors

 Last updated by: chrisdavidmills,